1. Field of the Invention
The present invention relates generally to the art of fabricating safety glass laminates and more particularly to the art of safety glass laminates with exposed plastic surfaces.
2. The Prior Art
Safety glass is a well-known term describing a glass-plastic laminate designed to reduce the severity of lacerative injuries. A plastic film is laminated to a glass sheet so that, upon impact sufficient to break the glass, the film adheres to the glass fragments, thus minimizing their dispersion. Commercially available safety glass is most commonly a multiple laminate of two glass sheets with an interlayer of polyvinyl butyral.
U.S. Pat. No. 2,047,253 to Brooks discloses that a safety glass laminate comprising a single glass sheet and a layer of plastic is more resistant to breakage by impact than a conventional trilayer laminate.
U.S. Pat. No. 2,120,628 to Reid claims bilayer laminates comprising a single glass sheet and a layer of polyvinyl acetal resin, but does not disclose a fabrication method.
U.S. Pat. No. 2,184,876 to Sherts discloses laminating glass to plasticized polyvinyl acetal using an outer layer of unplasticized resin covered by a flexible sheet plated with chromium as a parting material to facilitate release of the plastic surface.
U.S. Pat. No. 2,811,408 to Braley teaches generally the use of a copolymeric siloxane resin as a release agent on metal, wood, ceramic or plastic mold surfaces.
U.S. Pat. No. 3,509,015 to Wismer et al discloses fabricating bilayer safety glass by casting a polyurethane film between a glass sheet and a mold coated with a release agent to facilitate removal of the bilayer laminate from the mold. No particular release agents are suggested.
U.S. Pat. No. 3,808,077 to Rieser et al discloses fabricating bilayer safety glass by assembling a preformed plastic sheet between a glass sheet and a mold coated with a release agent. Suggested release agents include polyvinyl fluoride, polyethylene glycol terephthalate, organopolysiloxanes, and high silica content glass resins.
Whether a bilayer laminate is fabricated with a polymer cast and cured in place or a preformed thermoplastic sheet, the plastic surface to be exposed is pressed against a mold surface during fabrication. That surface, therefore, must have an optical finish, preferably equivalent to polished glass, in order to impart an optical finish to the plastic. For this reason, glass is a particularly suitable material for the pressing mold. In addition, a glass mold withstands the laminating process conditions, is simple and inexpensive to form, and has physical properties similar to the glass ply of the laminate so that distortion of the plastic during lamination is minimized.
However, the pressing surface must also have low adhesion, that is, good releasability from the plastic. Glass does not. Therefore, a release agent, a material which does not adhere to one or the other of the glass or plastic, must be disposed between the plastic and the pressing surface. While such commercially available release agents as polyvinyl fluoride and poly(tetrafluoroethylene) provide acceptable release properties, the surface presented to the plastic layer during lamination is not optically smooth, resulting in optical distortion of the plastic which is unacceptable.
The quality of the release surface is most critical in the fabrication of curved bilayer windshields for automobiles. Curved bilayer windshields have an outer glass ply the exterior surface of which is convex and an inner plastic ply the interior surface of which is concave. Therefore the surface of the mold which is pressed against the plastic must be convex. Thus during lamination the bilayer windshield is in effect wrapped around the mold making the release more difficult than the release of flat laminates.
The present invention discloses a method for coating the pressing surface of a glass mold to produce a superior release surface for use in making safety glass with an exposed plastic surface that is free of optical defects.